After Hull dominated the early possession, West Ham started to see more of the ball as the half progressed, but failed to create a clear-cut chance in the opening 20 minutes.
Diafra Sakho came close to setting up the opening goal for the visitors with just over half an hour gone, breaking from midfield before trying to put the ball through for Enner Valencia, but Michael Dawson turned it behind.
After another chance for Valencia, Hernandez had an opportunity down the other end after linking up with Nikica Jelavic, but his finish was straight at the goalkeeper.
The summer signing did open the scoring a couple of minutes later though, marking his debut for Hull by heading in from a Dawson cross.
Hernandez almost bagged a brace just before the break as he lashed the ball against the crossbar, while Jelavic was flagged as offside as he followed the ball in for the rebound.